In recent years, along with a tendency to high brightness and whitening of a light-emitting element such as a light-emitting diode (LED), a light-emitting device employing a light-emitting element has been used for illumination, backlights of various displays or large-sized liquid crystal TVs, etc. The substrate for mounting a light-emitting element, to mount a light-emitting element, is usually required to have a high reflectivity to efficiently reflect light emitted from the element.
Accordingly, it has been heretofore attempted to provide a reflection layer (optical reflection layer) on the substrate surface for the purpose of reflecting light emitted from a light-emitting element to a forward direction as far as possible. Further, as such a reflection layer, a reflection layer composed mainly of silver (hereinafter, referred to as a silver reflection layer) having a high reflectance is employed.
However, silver is likely to be corroded, and if it is left to stand in an exposed state, oxidation or sulfurization tends to occur on the surface of the silver reflection layer, whereby reflectance (optical reflectance) tends to deteriorate. Accordingly, it has been proposed to cover the surface of the silver reflection layer with e.g. a silicone resin, an acrylic resin, an epoxy resin or a urethane resin so as to prevent deterioration of the reflectance. However, by such a method, moisture or a corrosive gas is likely to pass through the resin or enter from the interface between the silver reflection layer and the resin, whereby it was impossible to sufficiently prevent the deterioration of the reflectance due to corrosion (oxidation or sulfurization) of the silver reflection layer.
Accordingly, in recent years, in order to prevent corrosion of a silver reflection layer, a method has been proposed to coat the surface of a silver reflection layer with a protective layer made of a glass (see e.g. Patent Document 1). With the glass-made protective layer, the sealing property is excellent as compared with a resin-made protective layer, further the light transmittance is high, and the quantity of light arriving at the silver reflection layer increases, whereby it is possible to obtain a high reflectance. Further, a glass is excellent in the thermal conductivity, and therefore in a case where a glass layer is provided as a protective layer for the silver reflection layer, it is possible to obtain a heat dissipating property higher than a case where a resin layer is provided.
However, in such a case where a glass layer is provided as a protective layer, deformation due to e.g. swell tends to occur during firing since a glass usually has a high fluidity in a non-fired state, whereby the flatness of the surface of a glass layer deteriorates. Accordingly, in a case where a light-emitting element is mounted on the glass layer, the contact area between the light-emitting element and the glass layer becomes small, whereby the heat resistance increases. Further, if irregularities are formed on the mounting portion, the light-emitting element is likely to be fixed with an inclination and damaged by subsequent wire bonding, and further an optical axis is also likely to be displaced.
In order to solve these problems, in recent years, it has been studied to provide a glass layer having a ceramics filler incorporated, as a protective layer for a silver reflection layer. When the protective layer is made of a sintered product of a mixture of a glass powder and a ceramics filler, the fluidity in a non-fired state is lowered, and the flatness becomes high. Further, in a case where the ceramics filler is incorporated in the glass layer, the reflection direction of light entering the glass layer from a light emitting element can be dispersed, whereby fluctuation in light distribution characteristics can also be reduced.
However, as shown in FIG. 5(a), in a case where a glass layer 53 containing an alumina filler 52 containing e.g. Al2O3 as the main component is provided on the silver reflection layer 51, silver ions migrate (migration) from the silver reflection layer 51 to the glass layer 53 in a firing step. Further, as shown in FIG. 5(b), silver ions migrated are localized on the surface of the alumina filler 52 and a periphery thereof, whereby the silver ion layer 54 at a high concentration is formed. Further, such a phenomenon is likely to occur that a part of the silver ion layer 54 localized on e.g. the surface of the alumina filler 52 is likely to be exposed on the surface of the glass layer 53. Further, as shown in FIG. 5(c), in a case where a light-emitting element is mounted on such a glass layer 53 and sealed with a sealing layer 55 such as a silicone resin, the silver ions (Ag+) at a high concentration, contained in such silver ion layers 54 exposed on the surface of the glass layer 53, are likely to be in contact with e.g. a platinum catalyst contained in a sealing layer 55 and thereby to be reduced to form Ag0, and as shown in FIG. 5(d), they are likely to cohere to be formed into colloidal particles. Further, there has been also a phenomenon of so-called silver coloring, that is, silver particles 56 (colloidal particles) thus cohered are colored at the interface between the glass layer 53 and the sealing layer 55 such as a silicone resin. As a result, reflectance was likely to deteriorate, whereby light intensity (brightness) as a light emitting device was likely to deteriorate.